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PLANT DESIGN AND ECONOMICS FOR CHEMICAL ENGINEERS

developing the individual components of the design. Each safety factor must be chosen on basis of the existing conditions, and the chemical engineer should not hesitate to use a safety factor of zero if the situation warrants it.SPECIFICATIONS

A generalization for equipment design is that standard equipment should be selected whenever possible. If the equipment is standard, the manufacturer may have the desired size in stock. In any case, the manufacturer can usually quote a lower price and give better guarantees for standard equipment than for special equipment. The chemical engineer cannot be an expert on all the types of equipment used in industrial plants and, therefore, should make good use of the experience of others. Much valuable information can be obtained from equipment manufacturers who specialize in particular types of equipment. Before a manufacturer is contacted, the engineer should evaluate the design needs and prepare a preliminary specification sheet for the equipment. This preliminary specification sheet can be used by the engineer as a basis for the preparation of the final specifications, or it can be sent to a manufacturer with a request for suggestions and fabrication information. Preliminary specifications for equipment should show the following: 1. 2. 3. 4. 5. 6. 7. 8. 9. Identification Function Operation Materials handled Basic design data Essential controls Insulation requirements Allowable tolerances Special information and details pertinent to the particular equipment, such as materials of construction including gaskets, installation, necessary delivery date, supports, and special design details or comments

Final specifications can be prepared by the engineer; however, care must be exercised to avoid unnecessary restrictions. The engineer should allow the potential manufacturers or fabricators to make suggestions before preparing detailed specifications. In this way, the final design can include small changes that reduce the first cost with no decrease in the effectiveness of the equipment. For example, the tubes in standard heat exchangers are usually 8, 12, 16, or 20 ft long, and these lengths are ordinarily kept in stock by manufacturers and maintenance departments. If a design specification called. for tubes 15 ft Ion& the manufacturer would probably use 16-ft tubes cut off to the specified length.

Thus, an increase from 15 to 16 ft for the specified tube length could cause a reduction in the total cost for the unit, because the labor charge for cutting the standard-length tubes would be eliminated. In addition, replacement of tubes might become necessary after the heat exchanger has been in use, and the replacement costs with 16-ft tubes would probably be less than with 15ft tubes. Figures 2-6 and 2-7 show typical types of specification sheets for equipment. These sheets apply for the normal type of equipment encountered by a chemical engineer in design work. The details of mechanical design, such as shell or head thicknesses, are not included, since they do not have a direct effect on the performance of the equipment. However, for certain types of of equip- I ment involving unusual or extreme operating conditions, the engineer may need

Utilities: Controls: Imul8tion: Tderances: Comments and draw@:

FIGURE 2-7 Specification sheet for sieve-tray distillation column.

to extend the specifications to include additional details of the mechanical design. Locations and sizes of outlets, supports, and other essential fabrication

information can be presented with the specifications in the form of comments or drawings.

MATERIALS OF CONSTRUCTIONThe effects of corrosion and erosion must be considered in the design of chemical plants and equipment. Chemical resistance and physical properties of constructional materials, therefore, are important factors in the choice and design of equipment. The materials of construction may be resistant to the